Use of $Z \rightarrow e^+ e^-$ events for ECAL calibration

The Z mass constraint in Z rightarrow e^+e^- events is a powerful tool to inter-calibrate ECAL regions, set the absolute energy scale and, ultimately, tune electron reconstruction algorithms. In this note a strategy to use Z events is proposed and tested with a sample of two millions fully simulated Z rightarrow e^+e^- events; the method avoids the inversion of huge matrices. Calibration results obtained with a sample corresponding to 2 fb^-1 of integrated luminosity are shown. A detailed study on the usage of different electron classes in Z events for calibration purposes is presented.